1. Field of the Invention
This invention relates generally to the art of processing and treating molten metal. More particularly, this invention relates to a new and improved coupling design for a molten metal processing system.
2. Discussion of the Art
Molten metal processing systems can usually be classified into several different types of systems. For example, degassing/flux injection, submergence and pumps are frequently used general categories.
Systems which fall into the degassing/flux injection category generally operate to remove impurities from molten metal. More specifically, these systems remove dissolved metals, such as magnesium, release dissolved gases, such as hydrogen, from molten metal, and through floatation remove suspended solid impurites. In order to achieve these functions, gases or fluxes are introduced into a molten metal bath which chemically react with the impurities to convert them to a form (such as a precipitate or a dross) that can be separated readily from the remainder of the molten metal.
Systems which fall into the submergence category generally operate to melt scrap metal, such as by-products of metal processing operations and aluminum beverage cans, in order to recover the scrap metal for productive use. In a typical submergence system, the scrap metal is introduced onto the surface of the molten metal and drawn downward or submerged within the molten metal where it is melted. In its melted form, the scrap metal is substantially ready for productive use.
The pump category can be further classified into three different types of systems including transfer pumps, discharge pumps, and gas-injection pumps. A transfer pump typically transfers molten metal from one furnace to another furnace. A discharge pump transfers molten metal from one bath chamber to another bath chamber. A gas-injection pump circulates molten metal and adds a gas into the flow of molten metal. Although the present invention is particularly well suited for use with a gas-injection pump or degassing system, it must be appreciated that this invention may be used with any rotor/shaft system, including but not limited to the systems mentioned above.
Known molten metal processing apparatus of the foregoing types typically include the common feature of a motor carried by a motor mount, a shaft connected to the motor at an upper end, and an impeller or rotor connected at a lower end of the shaft. A coupling mechanism is used to connect the upper end of the shaft to the motor. The components are usually manufactured from a refractory material, such as graphite or ceramic. In operation, the motor drives the shaft which rotates the impeller about its central vertical axis. The rotating impeller may serve any number of functions. For example, in a submergence system the impeller may draw molten metal downwardly to assist in the submergence of scrap materials deposited on the surface of the melt. In a pump system, the impeller may be contained within a housing to effect a pumping action on the metal. In a degassing/flux injection system, the impeller may introduce gas or flux into the molten metal via a passage located in the impeller body. Furthermore, the impeller may serve other conventional functions.
An important feature of impeller/shaft systems is the coupling mechanism which connects the upper end of the shaft to the motor. With reference to FIGS. 1A-1C, a series of shafts for known coupling designs are shown. Connecting an upper end of a shaft to a motor is most commonly achieved via a straight thread design as shown in FIG. 1A. The straight thread design includes an upper end 10xe2x80x2 having a plurality of external threads 12xe2x80x2. The threaded upper end is threaded into a coupling (not shown) extending down from a drive system (not shown). Like any conventional threading mechanism, the shaft is screwed into the coupling by turning it several times until it is tight and secure.
The straight thread design suffers from several shortcomings. During operation, the shaft of a rotor/shaft system is exposed to a number of forces, particularly shear forces resulting from cantilever loading. The straight thread design is a relatively weak coupling because the machining of the coupling causes stress risers in a ceramic or graphite shaft. This results in an increased potential for shaft failure which is obviously undesirable. Furthermore, when a shaft breaks, it typically breaks just below the coupling leaving little if any shaft extending from the coupling. Thus, there is little material to work with in order to unscrew the stub. In addition, because the resistance of the straight thread design is equal in both directions, it is extremely difficult to unscrew. In other words, a significant amount of torque is required to remove the stub. A chisel and hammer are generally required to accomplish removal.
Removing the stub with a chisel and hammer causes additional problems. The use of a chisel to remove the graphite stub may accidentally deform the threads in the coupling. Thus, the threads will have to be re-formed to their original dimensions. Such re-forming operations are time consuming and often result in shaft run-out. Moreover, because graphite is a soft material, the normal replacement of the shaft in a straight thread design may lead to graphite deposit in the coupling threads, resulting in binding and shaft run-out.
Additional problems arise when the straight thread design is used in connection with a degassing system. When used for such applications, the straight thread does not operate with optimal sealing properties which is an important characteristic for degassing systems to prevent leakage of the purge gas.
Two other known coupling designs have been introduced in order to overcome some of the problems associated with the straight thread design. The first is an electrode thread design, as shown in FIG. 1B. The electrode thread design includes a recess 14xe2x80x2 in the upper axial end of the shaft having a series of internal axial threads 16xe2x80x2. A male mating member (not shown) threads into the recess thereby connecting the drive system to the shaft. The second coupling is a tapered design which is shown in FIG. 1C. In this design, the upper end of the shaft is tapered and is configured to frictionally fit into a coupling (not shown). A male threaded shaft (not shown) extends from the coupling and fastens into a tapped bore 20xe2x80x2 extending through the central axis of the shaft.
The tapered design provides marginally increased strength to resist the lateral forces applied to the shaft. When the shaft does break for the tapered design, it is tedious to remove the portion of the shaft which still remains connected to the motor. The resistive force or required torque to remove the remainder of the shaft is so great that removal of a broken shaft can be done only with a significant amount of time and effort and a risk of damaging the coupling.
The electrode thread design also provides marginally increased strength to resist the lateral forces applied to the shaft. However, when the electrode thread design is used in connection with degassing equipment, it suffers from poor sealing properties which is an undesirable characteristic in such an application. Such a system does not seal well because of the large threads which are used. Additionally, because the threads are of a relatively soft material, they experience deformation which makes removal or backing off of the shaft extremely difficult.
Accordingly, a need exists in the art of processing molten metal to provide a coupling design for rotor/shaft systems which has optimal sealing properties, low run-out potential, relatively high strength to resist transverse forces, and can be easily removed at the end of its life or upon shaft failure. The present invention achieves such advantages and others.
In accordance with one aspect of the present invention, a coupling mechanism for a molten metal processing system includes an elongated shaft having a first axial end and a second axial end. The shaft preferably includes a passage extending longitudinally from a top surface of the shaft. The passage has a torque facilitating shape suited for accommodating a wrenching tool. At least one channel is disposed on an outer surface of the first axial end of the shaft. A coupling member connects the first axial end of the elongated shaft to a drive system. The coupling member has a cavity for receiving the first end of the shaft. The coupling member further includes at least one locking member disposed on a wall of the cavity that is adapted to cooperate with the at least one channel in a locking relationship. Typically the coupling is metal such as steel and the shaft is graphite or ceramic.
In accordance with another aspect of the present invention, a coupling device for a molten metal processing system includes at least one channel in a first surface and at least one locking member mounted to a second surface. The locking member is adapted to cooperate with the channel in a locking relationship.
In accordance with another aspect of the present invention, a molten metal processing system includes a drive system. A coupling member extends downward from the drive system. The coupling member couples a first end of an elongated shaft to the drive system. A passage having a torque facilitating shape extends longitudinally through the elongated shaft.
In accordance with another aspect of the present invention, a method for coupling a shaft of a molten metal processing system to a motor of the molten metal processing system includes forming a series of channels into an upper end of the shaft. The channels include having a first portion extending vertically downward from a top surface of the shaft and a second portion extending from the first portion at an angle greater than 90xc2x0 relative to the first portion. A series of locking members are provided on an inner surface of an annular wall of a coupling member which cooperate with the channels. The locking members are aligned with the channels. The shaft is then slid into the coupling member until the locking members have reached a bottom surface of the first portions of the channels. The shaft is turned so that the locking members travel partially through the second portions of the channels until the coupling member and the shaft are securely connected.
One advantage of the present invention is the provision of a coupling design that enables easy removal of a shaft stub which remains in a coupling member upon shaft failure.
Another advantage of the present invention is the provision of a coupling design that enables an operator to couple a shaft to a drive system in a quick and easy manner.
Another advantage of the present invention is the provision of a coupling design that provides optimal sealing properties for a degassing system.
Another advantage of the present invention is the provision of a coupling member that is formed into one piece which enables a shaft to be coupled to a drive system in a quick, easy, and efficient manner without having to deal with several tedious components.
Yet another advantage of the present invention is the provision of a coupling design which when machined reduces the occurrence of stress risers thereby increasing the ultimate strength of a rotor/shaft system.
Still another advantage of the present invention is the provision of a coupling device which reduces the potential for shaft run-out.
Still other benefits and advantages of the invention will become apparent to those skilled in the art upon a reading and understanding of the following detailed specification.